Sound projection system

ABSTRACT

A sound projection system comprising means defining a longitudinally extending acoustic channel which is acoustically open at a front end of the channel and acoustically closed at a rear end and at sides of the channel, a loudspeaker having a cone diaphragm operating into the channel at the rear end of the channel, and a longitudinally extending member mounted within the channel directly in front of, and in alignment with, the loudspeaker so as to restrict the free space within the channel, the cross-sectional area of said member varying along its length.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a sound projection system. One aplication ofthe invention is to a sound projection system which is particularlysuitable for use outdoors, and in other environments where it isnecessary to transmit sound over relatively long distances, in the formof a fairly narrow beam.

2. Description of the Prior Art

A large number of special types of transducers, and enclosures fortransducers have been proposed, which variously aim to provide improveddirectional characteristics, efficiency, and/or band width of response,and one of the more successful types of system is the well knownhorn-loaded type. In this type of system, the transducer feeds into therear of a flared horn, the characteristics of which are such that veryefficient conversion of electrical energy into acoustic energy can beobtained, as compared to a loudspeaker without such a horn.

Such horns do however suffer from a number of disadvantages in practice,in that in most cases they need to be rather large and of special shape,so that they are difficult to fabricate; and furthermore they are noteasily adaptable for different environments, since the ideal shape forthe horn varies with the environment, e.g. the size of room, in which itis being used. Further such horns are not as directional as may bedesired in some instances.

SUMMARY OF THE INVENTION

According to the invention there is provided a sound projection systemcomprising means defining a longitudinally extending acoustic channelwhich is acoustically open at a front end of the channel andacoustically closed at a rear end and at sides of the channel, aloudspeaker having a cone diaphragm operating into the channel at therear end of the channel, and a longitudinallly extending member mountedwithin the channel directly in front of, and in alignment with, theloudspeaker so as to restrict the free space within the channel, thecross-sectional area of said member varying along its length.

.Iadd.The diaphragm of the loudspeaker of the sound projection system ofthe present invention has a maximum longitudinal displacement at itscenter in response to an audio electrical signal. According to theinvention, the rear portion of the longitudinally extending member isspaced away from the diaphragm substantially in excess of the maximumlongitudinal displacement of the center of the diaphragm. .Iaddend.

Preferably the loudspeaker is mounted in an aperture in a baffle board,the channel extending forwardly of the baffle board. Preferably thebaffle board is at the front of an enclosure. Preferably the enclosureis a sealed enclosure.

Preferably the means defining the channel is a housing.

The enclosure and the housing may be formed as a single cabinet.

The housing may be externally parallel sided along its length.

Preferably the member is tapered towards the front end of the channel.

Preferably the member also tapers towards the loudspeaker. Preferablythe member tapers more abruptly towards the loudspeaker than towards thefront end of the channel.

The member may have a rounded rear end facing the loudspeaker.Preferably, however, the member has a flat rear end facing theloudspeaker.

A mounting for the member may be adapted to receive a range of suchmembers of various lengths, cross sections and profiles so that thesystem may be adapted to various working conditions by providingdifferent sound dispersion characteristics.

The member may be divided along its length into two or more separablesections so that the member of a chosen length, cross-section, profile,and front and rear termination configuration can be assembled from a setof components.

Because the effective internal cross-sectional area at any place alongthe length of the channel is governed, by the cross-sectional area ofthe member at that place, it is relatively easy to form a "horn" type ofsystem using this arrangement. Members of various cross-sections andprofiles can be simply turned out of wood or other suitable materials,so as to provide almost any desired variations of the effectivecross-sectional area of the horn along its length. Clearly, tomanufacture a housing with complex variations of cross-sectional areaalong its length would be very much more complicated.

BRIEF DESCRIPTION OF THE DRAWINGS

Two embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a longitudinal cross-section through a sound projection systemforming a first embodiment of the invention;

FIG. 2 is an end view of the system of FIG. 1 seen from the open end ofthe channel;

FIG. 3 is a cross-section through a sound projection system forming asecond embodiment of the invention; and

FIG. 4 is a fragmentary perspective view of the system of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and, in particular to the system shown inFIGS. 1 and 2 comprises a cabinet 2 of square cross-section, which isconstructed as rigidly as possible from a material which issubstantially "dead" acoustically, such as solid timber. A loudspeakerenclosure 4 at the rear end of the cabinet is provided with a baffleboard 6 facing the front end 8 of the cabinet which is open, and thebaffle board has an aperture 10 in which a loudspeaker having a conediaphragm is mounted. The joints of the enclosure 4 and the jointbetween the loudspeaker and the baffle board are made airtight so as toprovide a "totally enclosed" type of enclosure of fixed and relativelysmall voluem behind the speaker. The enclosure 4 is preferably filledwith acoustically absorbent material to provide internal damping inknown manner per se.

The part of the cabinet in front of the speaker is a housing 12, withparallel plane outer surfaces, and is provided with triangular fillets14 extending along the inside edges of the housing. The fillets are inthe form of isosceles triangles, each one having its base 16 in contactwith, and extending across one corner of the baffle board 6; and havingits apex 18 adjacent the corresponding corner of the open end 8 of thehousing. The housing 12 thus defines a longitudinally extending acousticchannel which is acoustically open at the front end thereof (that is, tothe right in FIG. 1) but is otherwise acoustically closed, moreparticularly at the rear end and at the sides of the acoustic channel,the loudspeaker operating into the rear end of the channel. The fillets14 provide the housing 12 with an internal cross-sectional area whichvaries along the axis of the housing 12.

A restrictor member 20 is axially mounted in the housing 12 so as toextend into the aperture 10 of the baffle board with its rear end 22close to the voice coil assembly 24 of the loudspeaker. The front end 26of the restrictor member 20 extends to the region of the front aperture8 of the housing:

The restrictor member 20 is generally bullet or "streamline" shaped,having a maximum cross-sectional area in its central region and taperingtowards both ends, more rapidly to the rear than the front. Fordifferent applications, it may be found desirable to use a restrictormember of a different central profile, or having differently profiledend portions, and the restrictor member of this embodiment may thereforebe formed in four sections 28, 30, 32 and 34 in FIG. 1. In this caseonly one of these, that is section 30 in the drawings, is fixedlymounted to the cabinet 2 by means of a pair of radially extending fins36 and the other sections are provided with screw threaded connectionsby means of which they may be attached to each other and to the fixedsection 30.

In order to ensure the smoothest possible sound wave travel along thehousing, the mounting fins 36 are preferably of streamlined section,their cross-section corresponding to that of the restrictor member, buton a reduced scale.

A prototype system which has been found to have excellentcharacteristics compared with an ordinary hornloaded system, beingnoticeably lacking in the forms of intermodulation distortion commonlyproduced by horns, and which manifests itself as a quality of"throatiness" in speech, has been built to the following dimensions:

Overall length of cabinet 12: 2' 6"

Aperture at open end 8: 12"square

Length of enclosure 4: 9"

Length of restrictor means 20: 22"

Diameter of restrictor means 20 at widest point: 5"

Diameter of restrictor half way between widest point and front end: 4"

Restrictor mounting fins 36, length along housing: 51/2"

Spacing between rear end of restrictor means and loudspeaker voice coildust cap: 1/2"

The loudspeaker used in this system was approximately 12" (12 inches) inoverall diameter. The system was constructed from 1/2" thick plywood. Itis believed that at least some of the improved characteristics of thesystem as compared to an oridinary horn, is that the transducer and theenclosure are more readily "coupled" by the arrangement of therestrictor member.

The system illustrated in FIGS. 3 and 4 is similar in many respects tothe system of FIGS. 1 and 2, and corresponding references areaccordingly used in FIGS. 3 and 4, but with the suffix "a", for thebaffle board 6a, the open front end 8a, the baffle board aperture 10a,the housing 12a, the restrictor member 20a, the front end 26a ofrestrictor member 20a and the mounting fins 36a.

The housing 12a is both internally and externally parallel-sided, havingno fillets corresponding to the fillets 14 of FIGS. 1 and 2.

The restrictor member 20a of FIGS. 3 and 4 is differently shaped fromthe restrictor member 20 of FIGS. 1 and 2, the restrictor member 20atapering more abruptly toward its rear end 40 from its maximum width andhaving a flat, instead of rounded, rear end, so that the forwardlytapering part of the restrictor member 20a can be positioned moreclosely to the loudspeaker than in the case of restrictor member 20. Theflat rear end 40 of restrictor member 20a is possible without causingstanding waves because .Iadd.of .Iaddend.the finite diameter of theloudspeaker voice coil 42 with its curved cover. The housing 12a is cladon the outside thereof with acoustic damping material 44, not shown inFIG. 4.

The baffle board 6a is not essential, provided the loudspeaker edge issealed acoustically to the rear end of the housing 12a. An acoustically"transparent" covering (not shown) may be placed over the front end ofthe housing 12a if desired.

.Iadd.The diaphragm of the loudspeaker of the sound projection system ofthe present invention has a maximum longitudinal displacement at itscenter in response to an audio electrical signal. As shown in thedrawing figures, especially FIGS. 1 and 3 thereof, and as indicated inthe description of the prototype system mentioned above, the rearportion of the longitudinally extending restrictor member 20, 20a isspaced away from the diaphragm substantially in excess of the maximumlongitudinal displacement of the center of the diaphragm. .Iaddend.

.Iadd.Further, the distance between the loudspeaker diaphragm and thesurface of the rear portion of the restrictor member may increase in thedirection parallel to the longitudinally extending restrictor memberalong the free space defined by the diaphragm and the rear portion ofthe longitudinally extending restrictor member. This is shown, forexample, in both FIGS. 1 and 3. Additionally, the sound projectionsystem may be constructed such that the point where the rear portion ofthe member becomes abruptly tapered toward the loudspeaker issubstantially forward of the forward edge defined by the partfrusto-conical volume. This is shown, for example, in both FIGS. 1 and3. .Iaddend.

.Iadd.Furthermore, the free space between the housing and thelongitudinally extending restrictor member may have a substantiallyconstant cross-sectional area between the points where the rear portionbecomes abruptly tapered and where the front portion becomes tapered.For example, this is shown most clearly in FIG. 3. .Iaddend.

What is claimed is as follows:
 1. A sound projection system comprising ahousing having a means defining a longitudinally extending acousticchannel, said channel having an acoustically opened front end and anacoustically closed rear end and acoustically closed sides; aloudspeaker carried at the rear of said housing, said loudspeaker havinga cone diaphragm defining a part frusto-conical volume and operatinginto the channel at the rear end of the channel.Iadd., said diaphragmhaving a maximum longitudinal displacement at a center of said diaphragmin response to an audio electrical signal.Iaddend.; and a longitudinallyextending member mounted within the channel directly in front of, and inalignment with, the loudspeaker to restrict the free space within thechannel, said member having a front portion, which is tapered towardsthe front of said channel, and a rear portion, which is tapered abruptlytowards said loudspeaker and projects into said part frusto-conicalvolume, terminating with a flat end facing the loudspeaker, said memberbeing supported by a strut fixedly attached at one end to the inner wallof said housing extending radially inward and attached fixedly to saidmember at the other end .[...]..Iadd., said rear portion of said memberbeing spaced away from said diaphragm substantially in excess of themaximum longitudinal displacement of the center of said diaphragm..Iaddend.
 2. The sound projection system as claimed in claim 1, whereinsaid rear portion is of part frustoconical shape.
 3. The soundprojection system as claimed in claim 1, wherein said housing isexternally parallel sided along its length.
 4. The sound projectionsystem as claimed in claim 1, wherein said housing is internallyparallel sided along its length.
 5. The sound projection system asclaimed in claim 1, wherein the longitudinal distance between ends ofsaid member along the surface thereof is greater than the length of saidchannel along the inside surface thereof.
 6. The sound projection systemas claimed in claim 1, wherein said member tapers more abruptly towardssaid loudspeaker than towards the front end of said channel.
 7. Thesound projection system as claimed in claim 1, wherein said member has aflat rear end facing said loudspeaker. .Iadd.
 8. The sound projectionsystem as claimed in claim 1, the distance between the loudspeaker conediaphragm and the surface of the rear portion of the longitudinallyextending member increasing in the direction parallel to thelongitudinally extending member along the free space defined by saidcone diaphragm and the rear portion of said longitudinally extendingmember. .Iaddend. .Iadd.9. The sound projection system as claimed inclaim 1, the point where the rear portion of said member becomesabruptly tapered toward said loudspeaker being substantially forward ofthe forward edge defined by said part frusto-conical volume. .Iaddend..Iadd.10. The sound projection system as claimed in claim 1, the freespace between said housing and said longitudinally extending memberbeing of substantially constant cross-sectional area between the pointswhere said rear portion becomes abruptly tapered and said front portionbecomes tapered.